1. Field of the Invention
The present invention relates to a method and apparatus for correcting the position of a horizontally movable member such as a ram and a quill of a machine tool and more particularly to a method and apparatus for correcting the position of a free end of a horizontally movable member deviating vertically from a reference position when the horizontally movable member is moved horizontally.
2. Description of the Related Art
Conventionally, there is known a machine tool in which a column has a vertically movable member such as a spindle head, and a ram (quill) as a horizontally movable member with a main shaft (spindle) on which a tool is mounted is movable horizontally on the vertically movable member. In this machine tool, there is the problem that if the ram is advanced or if a large-sized tool or attachment is mounted on the main shaft, the ram would be bent and thus its free end would hang down compared to a reference position due to the weight of the ram or tool itself, so that an error would be involved in machining.
In order to eliminate such a machining error due to the hanging-down of the free end of the ram, the position of the free end of the ram is conventionally corrected using a straightness correcting function provided optionally in a numerical control (NC) device.
However, there is the problem that this straightness correcting function can correct only for one shaft. For example, in an on-floor horizontal milling and boring machine in which the ram and the main shaft are both advanced, the straightness correcting function can only correct one of the ram and main shaft and cannot correct a possible deviation of the position of the other occurring when the other is extruded.
Therefore, as disclosed in each of Japanese Patent laid-open No. 57-114649, 60-9635 and 62-99037, a device is provided which corrects the tilt of the ram or spindle head with the pressure of a hydraulic oil and the balancing force of a balance weight concerned, namely, the centroid adjusting device. By using both of the centroid adjusting device and the straightness correcting function, both of the ram and the main shaft are said to be corrected with reference to position.
Since this centroid adjusting device corrects the tilts of the ram and spindle head, however, there is the problem that quantities of the correction to them are limited. Another problem is that various correcting devices are required to be provided additionally for adjusting the pressure of the hydraulic oil and the balancing force, so that cost would be high.